The measurement of particulate matter of effluent gas flowing in a stack or other exhaust conduit of such stationary sources as coal burning facilities, garbage incinerators, hazardous waste type incinerators, concrete plants, paper/pulp processing plants and the like, is important because of the relationship between particulate matter and adverse health effects. Particulate matter exiting a stack of such an industrial source disburses into the atmosphere where it is inhaled by humans. Suspended particulate matter is known to produce a variety of deleterious health effects when inhaled. Monitoring of the particulate mass and/or concentration of the effluent gas in a stack is, therefore, important for health reasons.
Particulate matter exiting a stack is made up of many regulated substances. Measurement of particulate matter mass can also be used as a surrogate measurement of these other regulated substances.
Accordingly, regulatory agencies around the world require the continuous measurement of particulate matter emissions from stacks. A disadvantage of all present continuous stack particulate monitors--opacity, triboelectric, acoustical, and beta attenuation--is that they do not directly weigh particulate and must be periodically calibrated using manual mass measurements.
Manual measurement methods are defined in terms of utilizing a filter medium to capture particulate matter while measuring the total volume of effluent gas which has been filtered at the stack temperature by the medium over a period of time. There are various approaches available to unambiguously determine the flow rate through the filter over time and, hence the volume of gas sampled. However, effluent gas often contains water which adds non-particulate mass to the filter medium. To accurately represent the filter mass, the uncombined water must be removed.
As a result, the current Environmental Protection Agency (EPA) reference method (Method 17) in the United States is a manual method that requires the removal of uncombined water prior to and following particulate collection. The manual method consists of: (1) filter equilibration under a predefined range of temperature and humidity conditions; (2) a pre-collection weighing of the filter; (3) the installation of the filter in the manual sampler and the obtaining of a representative effluent gas sample from within the stack; (4) the removal of the filter from the stack and the sampler, and post-collection clean up of the nozzle and housing (all particulate on the walls of the nozzle and filter housing leading to the filter medium must be collected as part of the sample); (5) post-collection conditioning under the same equilibrium conditions for the filter as performed in the preconditioning (this procedure removes uncombined water from the sample and filter medium); and finally, (6) post-collection weighing of the filter to determine the mass captured on the filter medium. Steps (1), (2), (5) and (6) are normally implemented in the controlled environment of a laboratory remote from the stack. Because of the filter and apparatus handling required, this manual measurement method contains many opportunities for measurement error. It is also very labor intensive, tedious and expensive. The method provides only an average particulate concentration for the sample period, and requires a great deal of care to give repeatable results because of the many inherent sources of error such as filter handling, transport, conditioning and weighing. Another disadvantage is that useful time history information, describing transients and stack stratification, is lost. An easier, faster, more repeatable technique would also allow for more accurate and frequent calibrations of the present indirect continuous emission monitors.
Obtaining a representative particulate sample from a particulate laden effluent gas stream generally requires that the sample be obtained isokinetically. That is, a particle traveling in the sample inlet must possess the same kinetic energy as a particle traveling in the effluent gas stream. Since kinetic energy is a function of only mass and velocity, mass being constant, isokinetic sampling requires that the particle velocity at the entrance of the sample inlet must match the particle velocity of the effluent gas. Industrial processes that produce effluent gas streams are continuously changing, resulting in continuous velocity changes over time. Also, the velocity profile across an effluent conduit or stack is not constant, resulting in required instrument flow changes during traverse sampling. Therefore, for a measurement to remain isokinetic, the instrument sample flow must be altered also. When instrument sample flow is altered to maintain isokinetics, pressure changes in the measurement device are experienced and may result in erroneous mass readings if not properly controlled or accounted for.
A need thus persists for a particulate mass measurement instrument which reduces many of the error sources associated with manual and isokinetic sampling, and provides more representative data, easier, faster and with enhanced accuracy.